O-17 NMR-STUDY OF SOLVENT EXCHANGE IN SOME AQUEOUS [CO(TREN)(X)(OH2 OH)](N+), [CO(CYCLEN)(X)(OH2/OH)](N+), AND [CO(N-MECYCLEN)(X)(OH2/OH)](N+) SYSTEMS (X = NH3, OH2/OH, N = 3, 2, 1)/

A O-17 NMR study (I = 1.0 M, NaClO4 or NaOSO2CF3, 25.0 degrees C) of s olvent exchange in labeled p- and t-[Co(tren)(NH3)OH2](3+), [Co(tren)( OH2)(2)](3+), [Co(cyclen)(OH2)(2)](3+), and [Co(N-Mecyclen)(OH2)(2)](3 +) ions (ca. 30% O-17) in aqueous solution has shown that loss of coor dinated OH2 is slow for all of the complexes (k(ex)/s(-1) = 1.1 x 10(- 5), 1.2 x 10(-5), 3.7 x 10(-5) (p-site)/8.7 x 10(-6) (t-site), 2 x 10( -4), and 2 x 10(-4), respectively). Values of k(ex) for solvent exchan ge in [Co(tren)(OH)(2)](+) have been determined as 9.7 x 10(-5) s(-1) (p-site) and 2.2 x 10(-7) s(-1) (t-site) Coordinated OH- in both p- an d t-[Co(tren)(NH3)OH](2+) also exchanges only slowly with solvent (k(e x)/s(-1) = 1.7 x 10(-4), and <1 x 10(-6), respectively), whereas excha nge of coordinated solvent in the aqua-hydroxo complexes [Co(tren)(t-O H2)(p-OH)](2+), [Co(cyclen)(OH2)OH](2+), and [Co(N-Mecyclen)(OH2)OH](2 +) is much more rapid (k(ex)/s(-1) = 0.03 (p-site)/0.01 (t-site), 12 a nd 15, respectively). Ligand-OH- exchange in these latter systems is i nterpreted as occurring via (indirect) S(N)1(CB)-type processes on the corresponding aqua complexes: [Co-(amine)OH](2+) reversible arrow [Co (amine-H)OH2](2+) --> exchange. This type of pathway is seen to be mor e efficient when leaving-group departure is synchronous with proton tr ansfer, and this appears to be more important for exchange in the cycl en and N-Mecyclen complexes where a reasonably acidic, adjacent syn NH proton is involved.